Perfluorinated ion exchange polymers are used in making ion exchange membranes and other structures such as electrodes for fuel cell membrane electrode assemblies (MEAs). A well known perfluorinated ion exchange polymer is perfluorosulfonic acid (“PFSA”) polymer, which is typically a copolymer of fluorinated ethylene units and sulfonyl-containing comonomer units. One such polymer, disclosed in U.S. Pat. No. 3,282,875, can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PDMOF) [CF2═CF—O—CF2CF(CF3)—O—CF2CF2SO2F], followed by hydrolysis and acid exchange. Another exemplary ion exchange polymer of the type disclosed in U.S. Pat. Nos. 4,358,545 and 4,940,525 has a side chain —O—CF2CF2SO3H. The polymer can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF) [CF2═CF—O—CF2CF2SO2F] followed by hydrolysis and acid exchange.
Liquid compositions of perfluorinated ion exchange polymers are known for use in the manufacture of ion exchange membranes, for use in membrane coatings containing conductive or nonconductive particles, and for many other uses. While such liquid compositions are sometimes referred to as solutions, the compositions are generally recognized as being dispersions of polymer particles. U.S. Pat. No. 4,433,082 discloses a process for preparing liquid compositions containing perfluorinated ion exchange polymers having sulfonic acid groups or a salt thereof. The preferred liquid compositions contain 20 to 90% by weight of water and 10 to 80% by weight of an organic compound such as a lower alcohol. Such liquid compositions are sold commercially under the trademark Nafion® by E. I. du Pont de Nemours and Company, and typically contain up to 10% by weight of a perfluorinated ion exchange polymer having sulfonic acid groups in a medium of approximately 30-60% by weight water, 15-30% by weight 1-propanol, 15-30% by weight 2-propanol, and less than 10% by weight (total) of miscellaneous components consisting of methanol, mixed ethers and other volatile organic compounds.
PFSA dispersions for different end uses are currently produced by distillation of a raw dispersion of PFSA polymer in a mixture of water and solvents such as ethanol or 1-propanol among others. The raw dispersion is formed in acid (SO3H) form from pellets of perfluorinated PFSA precursor resin in sulfonyl fluoride form through three process steps, namely, (1) a hydrolysis step, followed by (2) an acid exchange step, which is followed by (3) a dissolution step where the pellets are dissolved in the presence of solvents and water. The raw dispersions typically have a solids content of 18 to 23 wt %. The raw dispersion is then subjected to an ion exchange step to remove any metal ions in the dispersion. This ion exchanged raw dispersion must be further processed through extractive distillation and further dilution steps to adjust its composition to meet desired end product specifications such as % solids, % water, % 1-propanol, % ethanol, and viscosity among others. The distillation and dilution steps of this process have been difficult to control making such PFSA end product dispersions expensive to produce and difficult to reproduce.
U.S. Pat. No. 6,518,349 discloses spray drying of a dispersion of fluoropolymer in water or an organic solvent to produce a sprayable powder of friable particles of a non-fibrillatable fluoropolymer. The dispersion is spray dried with air or nitrogen at an inlet temperature of at least 290° C., but less than the melting temperature of the polymer, so as to produce a powder of water (or solvent) free granules that can be applied by electrostatic spraying techniques to the surface of articles such as cookware. Re-dispersion of the fluoropolymer powder is not disclosed.
US 2005/0171220 discloses spray drying an aqueous dispersion of a highly fluorinated polymer to obtain polymer particles that are re-dispersible in water. However, producing and containing the aqueous dispersion requires high temperatures and pressures as well as a special corrosion resistant reactor vessel made of an acid resistant alloy such as Hastelloy® or lined with an inert material such as glass or gold. The re-dispersions of the polymer particles may be treated with H2O2 to improve the color and remove undesirable odor.